The bottoms of ships, buoys and other structures that are submerged in seawater such as cooling water intake or discharge pipes are infested with organisms such as barnacles, tube worms, mussels and algae that attach to the surfaces of these structures and cause various troubles. It is routine practice to prevent the attachment of these marine organisms by coating the surface of the aforementioned items with antifouling paints. Antifouling paints are roughly divided into two classes. The antifouling paints of one class (A) employ antifoulants such as organotin copolymers and cuprous oxide that are capable of preventing the attachment of fouling organisms and have low solubility in seawater. Paints that employ organotin compounds as antifoulants are shown in Japanese patent Publication Nos. 21426/65, 9579/69, 13392/71, 20491/74, 11647/76 and 48170/77. The antifouling paints of the second class (B) do not employ any antifoulants and will not dissolve in seawater; instead, they use silicone rubbers that cure by the action of a catalyst or moisture to form a crosslinked film. For instance, an antifouling paint that uses a curable silicone rubber as a coating agent is shown in Japanese patent Publication No. 5974/78. An antifouling paint that uses a mixture of a silicone oil and an oligomer-like silicone rubber having a terminal hydroxyl group is shown in Japanese patent application (OPI) No. 96830/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). A mixture of a curable silicone rubber and a flowable organic compound that does not contain a metal or silicon is shown in Japanese patent application (OPI) No. 79980/78. A paint that serves to prevent the attachment of fouling marine organisms is also shown in Japanese patent Publication No. 3433/85 and this paint is composed of a mixture of an oligomer-like low temperature curing silicone rubber (such as ones available from Shin-Etsu Chemical Co., Ltd. under the trade names of "KE 45TS" and "KE 44 RTV") and liquid paraffin or petrolatum.
The antifouling paints of class (A) are further divided into two subclasses. In one subclass of such antifouling paints, the film-forming resin does not dissolve in seawater and only the antifoulant dissolves in seawater to prevent the attachment of marine organisms. The coatings formed from this class of antifouling paints exhibit the intended effect during the initial period of application but after the antifoulant on the surface of the coating is lost as a result of its dissolution in seawater, the antifoulant in the deeper area of the coating will gradually dissolve. However, the dissolution rate of the antifoulant decreases as the depth of the area in which it is present in the film of coating increases, and the antifouling effect of the paint becomes insufficient in the long run.
In the second subclass of antifouling paints of class (A), both the antifoulant and the film-forming resin dissolve in seawater. The antifouling effect is achieved solely by the antifoulant or by a combination of the antifoulant and the resin component (e.g., an organotin copolymer) and, in either case, the surface of the coating dissolves in seawater to continuously provide the antifouling film of coating with an active surface. Therefore, the coating formed from this type of antifouling paints is capable of maintaining the desired antifouling effect over a longer period than the aforementioned first subclass of paints (A). However, the effect of this type of antifouling paints is not completely satisfactory because the film of coating they form is consumed fairly rapidly. In addition, the antifouling paints that employ antifoulants have one common problem in that the antifoulants have a potential for polluting the sea and killing marine products such as fish and shells.
Antifouling paints of class (B) are designed to prevent the attachment of marine organisms by making use of the slipping property (low surface energy) of the silicone rubber coating. However, these paints have the following disadvantages associated with the mechanism of film formation that involves the crosslinking of silicone rubbers after paint application.
The first problem is associated with the curing of the applied coating. For instance, when an antifouling paint of the type described in Japanese patent Publication No. 3433/85 that employs a low temperature curing oligomer-like silicone rubber that cures by the action of moisture in air to form a film of coating is applied to a substrate, the crosslinking agent incorporated to control the curing condensation reaction of the silicone rubber is activated by the moisture or temperature of air to cause premature curing of the surface of the coating. This retards the curing of the deeper portion of the coating to produce an insufficiently cured film which is most likely to blister or separate from the substrate. Furthermore, the slow penetration of moisture into the bulk of the coating prolongs the time required to achieve complete curing of the coating.
If the antifouling paint of the type described above is applied in a hot and humid atmosphere, the hydrolysis of the crosslinking agent predominates over the crosslinking reaction and the resulting coating does not have a sufficient crosslink density to provide satisfactory properties.
In a dry climate, the amount of aerial moisture is too small to cause hydrolysis of the crosslinking agent and the applied coating will cure very slowly. In order to avoid this problem, catalysts such as tin compounds and platinum are sometimes used as curing accelerators but their effectiveness is limited in cold climates.
The second problem concerns the case of top-coating. In the usual case, the solvent in a paint for topcoating slightly dissolves the surface of the undercoat to ensure good intercoat bonding. However, in the application of the antifouling paint under consideration, the silicone rubber in the first applied coating cures to such an extent that the solvent in a paint for top-coating is not capable of dissolving the surface of the silicone rubber to provide satisfactory intercoat bonding.
The third problem is related to pot life. The actual coating operation is prolonged if the item to be treated is large in size or has a complex structure. In addition, the operation may be interrupted by unexpected rainfall. In view of these possibilities, antifouling paints having short pot lives present great inconvenience in coating operations.
The fourth problem is associated with storage stability. Antifouling paints, after being prepared, are stored until use and the duration of such storage sometimes extends for a long period. Therefore, the manufacture of paints that will cure by the action of moisture necessitates the filling of their containers with a dry nitrogen gas. In addition, once the container is opened, aerial moisture will get into cause curing of the surface of the paint or an increase in its viscosity. Paint that has undergone such changes is no longer suitable for use.